Left ventricular (LV) diastolic dysfunction and arrhythmias are important causes of morbidity and mortality in HCM. These abnormalities are believed to be in part due to myocardial fibrosis which frequently complicates HCM. Several studies indicate that pirfenidine can safely inhibit progression or cause regression of fibrotic lesions. We therefore propose to perform a six-month study that examines the ability of pirfenidone to improve LV diastolic dysfunction, exercise performance, and electrophysiologic abnormalities in symptomatic patients with severe HCM.